This disclosure concerns an invention relating generally to methods and apparata for measuring and testing engine characteristics, and more specifically to dynamometers for use with internal combustion engines.
The single-cylinder test engine (1CTE) has long been an important and widely-used tool in engineering and development of internal combustion engines. The 1CTE is typically a single cylinder, piston and head taken from a multi-cylinder engine (MCE), or having a design adapted from a cylinder, piston and head from a MCE, and which is used to simulate performance of an MCE on a smaller and simpler scale. Since the 1CTE has only a single cylinder, it is much easier to install and use modern laser diagnostics and other measurement instrumentation in a 1CTE than an MCE, thereby allowing more complete data collection regarding a cylinder""s fluid dynamics, heat transfer, thermodynamics, emissions and other characteristics. Additionally, owing to the simpler design of 1CTEs, they are much less expensive and time-consuming to build and modify when working out design challenges associated with combustion chamber shape, timing, or other geometric and thermodynamic issues, or to experimentally validate computation fluid dynamic results or predictions made on computers.
Along with these benefits of 1CTEs come many drawbacks. The most significant drawback is the difficulty in using 1CTEs to simulate low engine speed testing in MCEs. This difficulty arises primarily owing to two problems: speed variation over the 1CTE engine cycle preventing accurate MCE performance simulation, and resonant frequencies of the testing system arising at low engine speeds.
The problem of speed variation is most significant in four-stroke 1CTE testing. Since four-stroke MCEs are in common use in transportation applications, it would naturally be valuable to utilize 1CTE""s to simulate four-stroke MCEs. However, in four-stroke MCEs, each cylinder fires and provides kinetic energy to the crankshaft once every two crankshaft revolutions, with engine speed increasing during the expansion (power) stroke of the engine and then decreasing through the remaining three engine strokes of the combustion cycle. Since the multiple cylinders fire at different times in most MCEs, some cylinders provide increasing kinetic energy at the same time that kinetic energy output from other cylinders is decreasing, thereby resulting in relatively uniform engine speed. In contrast, during the two crankshaft revolutions between firings in a 1CTE, the 1CTE loses kinetic energy and slows down considerably. This problem is especially pronounced at low engine speeds (such as idle) because the time between cylinder firing increases. Thus, it is particularly difficult for a 1CTE to accurately simulate MCE operation at low speeds. To decrease this problem, a large flywheel (i.e., greater inertia) is typically added to the 1CTE crankshaft to store combustion energy as kinetic energy which is more uniformly released, thereby decreasing speed variation. Also, dynamometers with large polar moments of inertia are typically coupled to the 1CTE, further increasing the crankshaft inertia and allowing more uniform speed. While these measures prevent the 1CTE engine speed from decreasing significantly between cylinder firings, they also prevent accurate replication of the time-varying rotational dynamics that the 1CTE cylinder would experience if it was present in a MCE. (See, e.g., U.S. Pat. No. 6,212,945 to Moskwa and the references cited therein, which discuss dynamic engine models which take account of such time-varying dynamics.) Inertia addition additionally hinders useful study of transient engine operation (i.e., performance under changing speed conditions). As a result, the 1CTE does not accurately replicate MCE performance.
Resonant frequency problems arise because conventional electromagnetic engine dynamometer test systems have a resonant point at low engine speed. When the test engine is started and speeds up to the range desired for testing, it passes through the resonant point and causes excessive driveline vibration, which can skew test measurements and damage the apparatus if testing is maintained at or near resonant speeds. Use of the previously described flywheels can lower the natural frequency (particularly if more mass is added), but this further limits the transient response capability of the dynamometer. Similarly, vibration can be reduced by adding damping to the driveline by using flexible couplings, but this generally does not eliminate vibration problems at low speed.
Since researchers generally want the 1CTE""s operation to replicate what would be expected in the MCE, these drawbacks limit the utility of the information provided by the 1CTE. This is particularly true since the study of engine characteristics at low (idling) speeds, and during transient operation, is of significant interest in the study of fuel economy and emissions reduction, and the limitations of the 1CTE greatly hinder its usefulness for this purpose.
The invention involves apparata and methods for engine simulation and testing which are intended to at least partially solve the aforementioned problems. To give the reader a basic understanding of some of the advantageous features of the invention, following is a brief summary of preferred versions of the invention. As this is merely a summary, it should be understood that more details regarding the preferred versions may be found in the Detailed Description set forth elsewhere in this document. The claims set forth at the end of this document then define the various versions of the invention in which exclusive rights are secured.
A preferred version of the invention involves a dynamometer suitable for use with a single-cylinder test engine (1CTE), and which allows the 1CTE to simulate a multi-cylinder engine (MCE) by replicating the instantaneous engine dynamics present in an MCE. This is done by having the dynamometer not simply absorb the torque output of the 1CTE (as in standard dynamometers), but by having the dynamometer also provide a motoring torque input to the 1CTE which corresponds to the dynamic torques that would be delivered to the 1CTE from other cylinders in an MCE if the single cylinder of the 1CTE was actually present in a MCE. This motoring torque input is calculated in real time (or nearly so) by hardware and/or software-based calculation means which determine the motoring torque input from the other cylinders as if they were present alongside the single cylinder of the 1CTE. Thus, the 1CTE will act dynamically as if it were actually in a MCE, with instantaneous crankshaft speed being identical to that of an MCE throughout the engine cycle, providing a much more accurate simulation of an MCE. The dynamometer""s motoring torque input allows accurate simulation of MCE conditions at low (idling) speeds as well as at high speeds, thereby vastly enhancing the bandwidth at which a 1CTE may be used for MCE simulation. Additionally, the invention need not add significant inertia to the 1CTE, thereby allowing accurate study of transient engine operation.
The invention is therefore of significant benefit to engine research efforts because it allows the use of a simple and relatively inexpensive 1CTE to simulate MCE performance. Because only the one cylinder of the 1CTE is actually tested and the other cylinders of the xe2x80x9cvirtualxe2x80x9d MCE are simulated by a dynamic model, the configuration of the virtual MCE can be rapidly changed by changing its modelxe2x80x94for example, to study individual cylinder effects from various engine configurations, and/or to determine the effects of a different number of cylinders. The 1CTE can also be effectively connected to an entire virtual powertrain and/or other components to study any effects from dynamic coupling of these systems, or from transient operation.
While standard engine dynamometers are electromagnetically driven, the dynamometer used in the invention is preferably hydraulically driven, which enhances the ability to instantaneously (or nearly so) provide the calculated motoring torques to the 1CTE. The rapid response of the hydraulic dynamometer is further enhanced by controlling it with high-speed servo-valves as described later in this document. The beneficial rapid response of the hydraulic dynamometer has two ramifications.
First, the ability to rapidly load the 1CTE with the calculated torques allows the invention to simulate MCE performance across a far greater frequency or bandwidth of operating speeds than a 1CTE could do alone. In particular, simulation of MCE dynamic performance at idling speeds can now be performed with a 1CTE. A hydraulic dynamometer also enhances the dynamic range of a 1CTE since its low inertia provides a resonant frequency for the 1CTE/dynamometer system which is well above the standard operating speed range of the 1CTE.
Second, the rapid response of a hydraulic dynamometer allows use of the invention in transient or rapidly changing speed/load conditions. Since most current 1CTE testing apparata use flywheels and dynamometers having large polar moments of inertia, it is extremely difficult (and often effectively impossible) to use them for simulating transient or rapidly changing speed and load conditions. The low inertia of a hydraulic system allows instantaneous (or nearly so) response, and therefore a hydraulic dynamometer is able to rapidly conform itself to transient conditions.
The invention therefore allows simulation and testing apparata and methods which provide much more representative simulation of MCE performance than is believed possible with state-of-the-art 1CTE testing apparata at the time this document was prepared. Preferred versions of the invention allow the ability to test a 1CTE throughout the complete expected engine speed range of the actual MCE in which the cylinder of the 1CTE will be used, with accurate replication of the instantaneous dynamic operation and speed trajectory that would be expected in the MCE throughout the engine cycle. This aspect of the invention is particularly advantageous with respect to testing at idling speeds, since present 1CTE testing apparata known to the inventors simply do not allow a 1CTE to accurately simulate MCE performance at these speeds.
Further advantages, features, and objects of the invention will be apparent from the following detailed description of the invention in conjunction with the associated drawings.